This invention relates to a mobile crane with an overload protection according to the description herein.
In the overload protection of cranes, a limit curve usually is stored now for different crane configurations, which is monitored for exceedance during crane operation. Parameters relevant for crane safety include for instance the component strength of boom systems, hoisting cables, slewing ring, adjusting cylinders, mechanical connections etc. on the one hand and the static stability of the crane on the other hand. For each of these parameters, limit criteria exist, the minimum of which forms said limit curve, which is stored in a memory unit of the crane and which is monitored for exceedance during crane operation.
A disadvantage of such overload protections consists in that for every conceivable crane configuration and every possible supporting position including the extendable and retractable props a separate limit curve or lifting capacity table must be stored, which is comparatively expensive.
According to DE 20 2006 017 730 A1 it now is provided that individual limit curves or limit values are stored in the memory unit for various parameters of the crane, which must not be exceeded, or only by issuing a signal, in order to ensure the safety of the crane operation. In contrast to the aforementioned prior art, individual limit curves, for instance the admissible lifting capacity, thus are stored for the parameters boom strength, hoisting cable strength, static stability, etc. The means to ensure crane safety are configured such that they monitor these individual limit curves or also limit values of the various parameters for exceedance or approach. The advantage of this solution already has consisted in that the quantity of data to be stored or the size of the lifting capacity table can be reduced, since limit criteria need not always change with every change of a configuration of the components.
From DE 10 2005 035 729 A1 a method for operating a crane is known, whose admissible lifting capacity depends on one or more variable parameters. Accordingly, it is provided that the variation of the parameters is performed such that the speed of parameter variation is reduced continuously or incrementally, before a parameter value is reached, at which the admissible lifting capacity corresponds to the actual lifting capacity. Hence, there is no sudden or abrupt stop from the full speed of adjustment, but a continuous or incremental deceleration.
Finally, a mobile crane with extendable and retractable props for supporting the mobile crane is known from DE 10 2007 055 535 A1. Here, it is proposed that the mobile crane includes detection means for detecting the supporting forces in the supporting cylinders and a control means which is connected with the detection means and is configured such that it controls the extension and/or retraction of the supporting cylinders in dependence on the supporting forces or parameters detected by means of the detection means.
With the known overload protection, a mobile crane hence already can determine the length of the boom, the outreach, the angle of the boom and other operating parameters in dependence on the suspended weight, wherein the various strength values in accordance with the lifting capacity table and also the static stability are considered for a specific condition of the support. In real use, however, it frequently occurs that the props of the support of the mobile crane cannot be moved into the defined desired positions. For instance, insufficient space conditions during erection of the mobile crane are responsible for this situation.